Which Frequency Is Running In Each Mode?

[Steve Herschbach]

I find the whole George Payne way of conceptualizing things to be rather out of date myself. That was back in the day when only one thing mattered - detecting coins. Silver coins in particular. So he was looking at frequency, and most importantly, coin size targets.

If you do that, fix target size, you get the false idea that frequency corresponds to type of metal. Nickels respond here. Gold coins here. Copper coins here. And the biggie, silver coins here. That's how the first coin discriminators were conceived. But it has also lead to this mythology that frequency corresponds to metals. Gold is high frequency, silver low frequency.

No, it's not. There is no correlation between frequency and type of metal if you do not fix the size at some artificial limit. In fact, gold ranges from ground readings all the way to so-called silver readings. If you fix the metal type, frequency corresponds to size. Low frequency big gold, high frequency small gold.

There was also ground to deal with, and ground reacts less well at low frequencies, so a double bonus for silver hunters. You might think it is low frequency working better with silver. But you might also think of it in terms of the detector simply being better able to see the silver, for not seeing the ground.

It's all about conceptualization, and you can conceive of the same thing from different angles. I consider the old George Payne way of looking at things as obsolete from my perspective. It really was only something that worked well in the United States, and only because of an accident in our coin size and metal types. It allowed a scale to be created that worked well with silver coins and nickels, while knocking out a lot of trash items. In most other countries, our target id scale is worthless because their coins do not fit our classic scale.

I detect gold. I think in different terms entirely. For me frequency does two opposing things. Higher frequency is better for small targets. Small gold, small silver, small copper..... small stuff. But high frequencies also enhance ground, and especially, hot rock responses. The two effects offset each other, and can reverse things if ground is severe enough.

This also totally applies not to nugget hunters like myself, but almost anybody hunting coins and relics under any situation but the classic U.S. silver coin regime. Let me explain.

So I want to find gold nuggets. I must first think about the nugget size that I am looking for. I can look for the more common small gold, or the rarer large gold. If I want tiny gold, I usually want a high frequency detector, the higher the better. Now, here is the kicker. High frequency does just fine on large gold also. In fact, high frequency just detects well on everything - in the air. So air test a Gold Bug 2 on things, and it is amazing.

Unfortunately, the high frequency also "lights up" the ground to an amazing degree, and it is hard to get good depth on anything at very high frequencies. The signal attenuates rapidly in the ground, and the worse the ground is (more magnetite in general), the faster the depth drops off. Hot rocks that never responded at low frequencies are now everywhere at high frequencies.

Lower frequency starts looking better not just because it does better on large targets, but just as much because it is less reactive to the ground.

The 71 kHz Gold Bug 2 is an amazing detector. I can find pinhead gold with it. The big caveat is that in most nugget ground it has low penetration, and is very poor on large nuggets at depth. Not because it air tests poorly on large gold (it tests great), but because the ground sucks up the signal. 71 kHz is great for small gold, and even large gold in the mildest soils, but in bad ground it has poor depth, and makes hot rocks a real issue.

If I am looking for large gold at depth, I might very well use a lower frequency VLF in the old days, just as much because it is responding less to the ground as anything else, allowing large gold to be more easily found at depth. For my purposes, a PI detector for a long time was just a high power, super low frequency detector. Huge punch on large gold, with minimal ground response. So PI took over early on from the VLF low frequency, large nugget detectors of the time.

I mentioned relic detecting and coins in other countries. If you detect Europe, our U.S. coin scale is garbage. It's not "low frequency = silver." Over there silver can be all over the target id map. Huge silver coins. Or tiny silver coins. Or small coins hammered thin as foil. Or those hammered coins cut to make change. Silver under those circumstances occurs anywhere on your target id scale from ground to the highest reading, 0 to 100. It all just depends on the size, with a little ground effect tossed in to drag things down.

So in Europe, if you want to chase tiny silver cut coins, or very small gold coins, higher frequencies work well, whether it is gold or silver. The metal does not matter. It is size that matters. Relic hunters see the very same thing. High frequencies find the small bits, regardless of what they are - worst fact being tiny ferrous.

I long ago tossed the frequency and metal thing in my garbage can. Here is reality. High frequency will help you with smaller targets, but also make dealing with the ground harder. Low frequencies simply have less ground and hot rock response, and also less tiny trash stuff response, making them better if you want want to focus on larger targets, like coins or rings.

In my lifetime experience there is a crossover point for gold, and going too high enhances tiny gold nuggets, but also loses depth due to ground issues. A sweet spot develops around 50 kHz, which White's chose ages ago in the Goldmaster II, as being great for small gold nuggets, while still retaining punch in bad ground on larger gold nuggets. Minelab rediscovered this with the Gold Monster, and went with 45 kHz for this very reason. They found pushing high did better on tiny stuff, but the cost in larger heavier gold was not worth it to serious nugget hunters in bad Australian type ground.

If I was hunting tailing piles for ounce type gold nuggets, it is hard to beat a 15 kHz type detector, just like that ancient 15 kHz Garrett Groundhog circuit, that was at the time a high frequency, but in retrospect was a great large nugget lower frequency. The White's MXT at 13 kHz is superb on large nuggets in trashy locations.

If you are in Europe, that 15 kHz sweet spot applied for a long time, but more recently people have discovered the benefits of higher frequencies on these tiny cut silver and small gold coin finds.

Pulse Induction did serve as super low frequency for a long time. You gave up small gold to get big gold as deep as possible. The lack of ground response allows use of extra large coils. It is interesting to me that as newer PI detectors are pushed to get more sensitive to small gold, that ground and hot rocks have also become more problematic. The newest PI nugget hunters suffer from hot rock responses you never saw on the old PI models. PI is getting more like VLF over time.

So Billy, does Minelab put all this in Multi-IQ processing? Of course. But not in the way you think. They think more like me. It's every bit as much about ground, and saltwater, and even EMI, and what you do not want to detect, more so than metal types. A primary choice is saltwater - that forces a low frequency mix simply to avoid the salt response. Which, as I seem to have explained to beach guys a million times, also knocks out small gold responses.

For large coin detecting a lower frequency mix gives clean responses on larger targets like U.S. coins and rings, while getting less ground response, fewer hot rocks, and far less tiny trash signaling. It is not targeting silver coins per se, just larger stuff. For tiny items, gold nuggets, small hammered silver coins, a higher frequency mix works well, but you will deal with more ground and hot rock response, more tiny trash.

Forget metal type. Think size and ground, including saltwater, and hot rocks. As you increase frequency, everything responds better, and small items that respond poorly or not at all at low frequencies will do better. Ground, saltwater, and hot rock signals also increase with frequency. The first cut off is at saltwater. To work there, you must have a lower frequency mix to eliminate salt signal, and you lose all tiny stuff as well, tiny aluminum, tiny gold. This can also do very well on large targets in any ground.

The teens are really nice for general detecting, right on the edge of the salt range. 12 kHz - 15 kHz hits really well on most desired detecting targets, while not being overly sensitive to ground and the tiniest trash targets.

40 - 50 kHz is a sweet spot for gold nuggets and all really small targets, like the smallest cut silver coin, targeting the sub-gram range kind of stuff with some alleviation of ground and hot rock issues that develop at extreme frequencies.

You get up above 50 kHz and you really are just surface skimming for the tiniest bits. Depth just drops off rapidly due to the ground, and so this is specialty range for the smallest targets.

Multifrequency changes none of this, and making a machine that found everything best at all frequencies just gives you a detector that reacts to everything and finds nothing. It is about picking a few divergent frequencies that when differentials are applied, can add extra target information. This is as much about ground as anything else. The classic is the salt beach, where you want to notch out both salt response and ground response. Single frequency can't get you there except in crudest form, eliminating both, while losing a lot of gold. Using two frequencies lets you notch back in some gold jewelry missed by eliminating both ground and salt with a single frequency. 

Looking at two frequencies that are close together is a waste of time and processing power. The target and ground response is the same. But pick two very divergent frequencies, and you will see differences in size response and ground response. This whole idea of having a detector look at and analyze 100 frequencies simply makes no sense, and reveals the nonsense we have been fed for ages about more frequencies being better. Again, there are only a handful of gross frequency ranges that really matter.

Under 10 kHz = find U.S. large coins well, minimal small trash and ground responses, few hot rocks. call this Park Mode, with a special subset that tunes out salt, called Beach Mode

15 kHz plus or minus great on a large range of small to large targets, while still not being overly sensitive to ground and very tiny trash. Call this Field Mode, but really its just best all around mode.

40 - 50 kHz is great for sub-gram targets, but will make dealing with ground and tiny trash problematic. Let's call this a Gold Mode.

70 - 80 kHz is basically surface skimming for pinheads, max hot rock and tiny trash response. Pinhead Mode? 

Again, multifrequency really just adds better ground and target id capability for cleaner, more accurate responses across the board. It's not some magic about finding all targets best at all frequencies by lumping them all together. Most frequency discussions simply miss the reality of what is going on, and what is being achieved by going multifrequency.

I will say it one last time. Think of frequency, whether single frequency, or a mix of frequencies, in terms of the desired target size, offset by the added ground/salt response. Think of the target id scale as a size scale, low numbers are small targets, high numbers large targets. Forget aluminum or silver or gold. Aluminum responds anywhere on the scale. So does gold. Small foil a low single digit, and aluminum can like a silver quarter. Pick your frequency mix and your target id numbers to match the size of the targets you are seeking, and life will get easier. And quit thinking of multifrequency in terms of finding more targets better the more frequencies you use. Nonsense, just marketing nonsense.

[Steve Herschbach]

I like this image because it is one of the only ones I have seen that illustrate the effect on size instead of type. Minelab tried to take credit here, but it really was White's that found the 40 - 50 kHz sweet spot for small gold.

Again we have marketing at work. This frequency range is great for many small gold locations, while still doing well on large gold. But it is the gold on site that matters. If the location is old bucket line tailing piles, then there may be no small gold at all. Just larger oversize nuggets, say 1/4 ounce or larger, with main hope a multi ounce nugget. Then this chart is simply wrong, and that 18 kHz detector is now the winner. Conversely, what is there is nothing but 0.1 gram and smaller gold? Not that unusual actually in the goldfields. Now that 71 kHz machine reigns supreme. You have to know enough to read between the lines when it come to marketing, and in this case it is really telling you lower frequency for larger stuff, higher frequency for smaller stuff. Again, does not matter if it is silver, copper, or gold. It's all about size.

[GrievousAngel]

8 hours ago, Geotech said:

The use of the term "-3dB frequency" is misleading if you know what that term usually means in electronics. It's not a -3db frequency at all, rather it's just the frequency where you get a maximum R-response. A picture is worth 900 words so...

Suppose we're talking about a copper coin of some kind. At 1kHz it has a small response magnitude at a phase just past 90°. Increase the frequency to 2kHz and the magnitude and phase both increase. Again for 5kHz, 10kHz, and 20kHz. The magnitude and phase always increase with increasing frequency. Seems like the target will be easier to detect at 20kHz, that's a big response.

Except... we don't look at the target magnitude. Instead, we only look at the R-response. The reason is that ground is phase-adjusted to lie along the 0° axis which means the X-response may have a lot of ground signal in it while the R-response has none. This means that only the R-channel is clean and useful for initial target indication. (When you listen to a threshold-based detector you are listening to the R-channel. If you hear ground noise then it's because the ground phase isn't properly adjusted and some of it is getting into the R-channel.)

In this example the peak R-response is at 5kHz. This also corresponds to one-half of an eventual maximum X-response the target could possibly generate. Payne calls it the half-power/-3dB frequency which I don't care for because it doesn't correspond with the traditional use of those terms. Some people call it the resonant frequency which is even worse, there is no resonance at all. It's just a result of only using one-half of quadrature demodulation.

In any case, it is true that (everything else being equal) a 5kHz detector will detect this particular coin better than at other frequencies.

I agree. Yes it can be confusing for sure, as the terms are often used loosely. Due to time and avoiding over-stating an accepted law in a more casual setting, you have to be careful how one might explain a concept.

Yep, it is the 'special off-set' frequency at which the R-response component is at maximum level (magnitude) in which the detector receives and processes. Conceptionally I think of it as shifting the center frequency intended to be transmitted, up or down so it's at maximum magnitude when received. In terms of electrical power, -3db frequency point is called the half-power level, in which maximum power transfer is achieved via  impedance matching. This law is also critical in microwave waveguide design, i.e., used in many engineering fields.

My definitions and concepts were taken directly from the articles below. I should have referenced the sources in more detail.

The article Signal Frequency And Multiple Frequency as found on this site, https://detectorprospector.com/forums/topic/3193-selectable-frequency-and-multiple-frequency/ was the entry point for my thread. This article also referenced George Payne from his article found at http://jb-ms.com/Baron/payne.htm (2002).

I would suggest reading these articles if not already done so. I certainly improved my understanding of the science behind VLF frequency selection, single or multiple.

Cool stuff! Billy

 One of the experts in this arena is George Payne as referenced in

[GrievousAngel]

10 hours ago, Chase Goldman said:

Billy,

Here's what you do.  Search this forum for any post by "Geotech". He actually posted in one of your threads. That's Carl Moreland.  He is a design engineer who has worked for Whites and now works for First Texas.  Suck up any nuggets you can from his posts..

Then hop on over to his site.  If you ever want to build your own detector or just want to nerd out on the various technologies and scientific principles used for metal detecting (Faraday's law, induction balance, pulse induction) you can find the subject matter discussed in excruciating detail on the references pages and forum there. 

Finally, grab a paper or electronic copy of Carl's book, "Inside the Metal Detector", co-authored with George Overton.  Among a host of other topics about metal detector principles and technology, they describe how operating frequency affects target detectability in depth.  What you will find is there is no singular answer to the question you are seeking because the variables in play are simply too complex.  I get the impression you are looking for the optimal frequency to detect a silver Roosevelt dime vs. the optimal frequency to detect an 1849 $1 Gold piece vs. the optimal frequency to detect a King George III Copper.  The answer is a less than satisfying, "It depends".  And the best you can really do is speak in generalities.  For example, if you want to detect high conductors (e.g. silver coins) at depth go low in frequency (say 4khz), you want gold jewelry then 20 khz is a good neighborhood.  Then you need to consider ground effects that tend to attenuate higher frequencies and EMI which tends to be more prevalent at lower frequencies and you find that metal detecting is not about finding the -3dB half-power reactive component sweet spot but is really about balancing and trading off all these competing effects to give you the best chance to detect your particular target of interest.  HTH

Ahh- Carl beat me to it.  Thank you!  I was wondering when you were going to chime in.

Thanks for leads. I will knock on their doors soon. It is fortunate to be able to connect with others that truly have a 'meaningful understanding' of a given topic. Top-Drawer Experts!

Later, Billy

[GB_Amateur]

2 hours ago, Steve Herschbach said:

High frequency will help you with smaller targets, but also make dealing with the ground harder. Low frequencies simply have less ground and hot rock response, and also less tiny trash stuff response, making them better if you want want to focus on larger targets, like coins or rings.  (emphasis mine)

Thanks for another one of your classic, excellent long, detailed explanatory posts, Steve!  But I'm left with one confusing thing.  The Fisher Gemini 2-box T/R detector (~90 year old design by the original and founder, Gerhard Fisher himself, but still being sold today with a bit of modernization ?) uses 82 kHz operating frequency.  How does that squeeze its way into all of this?

[GrievousAngel]

Steve and the rest of us . . . 

Without any doubt, this site is the 'Top-Drawer'  source of accurate information, in all aspects of metal detecting - technical to use in the field! I'm totally impressed. There are so many knowledgeable detectorist hanging out on this site, who are eager to share information. 

I have an OK engineering background, but I'm short on actual use in the field. 

Thanks to all,

Billy

[Steve Herschbach]

25 minutes ago, GB_Amateur said:

Thanks for another one of your classic, excellent long, detailed explanatory posts, Steve!  But I'm left with one confusing thing.  The Fisher Gemini 2-box T/R detector (~90 year old design by the original and founder, Carl Fisher himself, but still being sold today with a bit of modernization ?) uses 82 kHz operating frequency.  How does that squeeze its way into all of this?

Good question. I think it's just that it is in effect just a huge coil, so huge it is insensitive anything smaller than a baseball, and ground also. It certainly does not act high frequency when it comes to what sets it off, targets and ground alike. It acts low frequency though we know it is not. Just very low gain maybe? We are talking quite literally about the oldest and simplest detector design ever sold commercially. Interesting, but I think more of apples to oranges in a way. Carl would know.

[GB_Amateur]

8 minutes ago, Steve Herschbach said:

Carl would know.

Presumably you're referring to Carl Moreland.    I only bring up that obvious point because I confused the issue by giving the name of the inventor and company founder as 'Carl Fisher' when his actual first name is/was Gerhard.  Gerhard probably knows the answer, too, but I don't think he's a member of this forum anymore. 

[Steve Herschbach]

8 minutes ago, GB_Amateur said:

Presumably you're referring to Carl Moreland.    I only bring up that obvious point because I confused the issue by giving the name of the inventor and company founder as 'Carl Fisher' when his actual first name is/was Gerhard.  Gerhard probably knows the answer, too, but I don't think he's a member of this forum anymore. 

I missed that. You said Carl Fisher? No, it's Gerhard. But yes, I mean Carl Moreland (Geotech)